Position sensing devices



SePt- 26, 1961 K D. J. MYNALI. 3,002,104

POSITION SENSING DEVICES Filed May 27. 1959/ United States Patent Claims priority, application Great Britain, June 4, 1958 Claims, (Cl. 307-43) This invention relates to position sensing devices as used for sensing or detecting the instantaneous relative positions of two relatively movable parts, and has an important application in connection with the determination` and/or control of the positioning of relatively movable parts of a machine tool.

Position sensing devices are known which depend for their operation on the mutual inductance existing between helical conductors which pass round closely spaced, facing, cylindrical surfaces on respective members movable with respect to each otherlengthwise of the vaxis of the helices, the mutual inductance varying cyclically as a result of such relative movementr of said members.

The present invention provides a device which involves a capacitive, rather than inductive, effect and which possesses certain advantages over the inductive devices as will be pointed out later.

According to the invention in its broadest aspect, a position sensing device comprising a first member having an inwardly or outwardly facing circumferential surface provided with at least one continuous, electrically conductive path passing round and progressing along the member in a manner analogous to that of a helix round a cylinder, together with a second member arranged for relative longitudinal movement with respect to the first member and defining a surface facing and substantially conforming to said'surface of the first member over at least a portion of the circumference thereof, which second member is provided with at least three separate .continuous conductive paths at least portions of each of which, located on said surface-of the second member and substantially conforming in shape and spacing to corresponding portions from adjacent turns in said path `on the first member, are disposed so as to have sensible electrical capacitance between them and the path on the first member, the arrangement being such that relative 3,002,104 Patented Sept. 26,. 1961 ICC' In the inductive devices, eddy current effects must be considered and minimised since the current paths lie in the magnetic field. Such considerations do not arise with the capacitive device, because the conductive paths are required to form equipotential boundaries of the (electric) field. Consequently there is a greater freedom of choice as regards the material and form of the conductive pathsin the capacitive device, thereby permitting the use of metals and manufacturing techniques which would not be suitable for the inductive devices.- A further advantage is that the conductive paths of the inductive devices have to carry appreciable currents (for instance of the order of 0.1 to 1.0 amp.) whereas in the capacitive device the currents are negligible. lIt is therefore easier to make satisfactory, long-life, electrical connections.

Finally, with inductive devices there are likely to be significant stray capacitance effects which contribute to error, and care has to be taken to minimise these effects. In the capacitive device, the capacitance is nota stray'f quantity and there is no significant equivalent stray inductance effect.

In further. considering the invention and the manner in which a position sensing Vdevice conforming thereto may be used, reference will be made to the accompanying drawings in which:

FIGS. l and 2 respectively illustrate possible forms of i the first and second members, as above identified, ofa

movement between the members in the said direction will produce cyclical variation of the capacitance between conductive paths on different-members.

- Where the first member, which is preferably but not Vnecessarily cylindrical, has more than one electrically `device of the invention has certain advantages over inductive devices. For instance the capacitive device absorbs much less, in fact almost negligible, electric power as compared with inductive devices which present appreciable resistance to an exciting source. Such virtual non-dissipation of power in the capacitive device is `advantageous in that the design of the excitation cir- Icuits can be simpler, and in that local heating effects ,are avoided which, by causing differential expansion, could have an adverse effect on the accuracy of the device in circumstances where utmost accuracy is required. Moreover it has been' found that precisely controlled 4voltages for the excitation of the capacitive device can be obtained from simpler circuits than those required ufor a corresponding inductive device.

position sensing device according to the invention, the second member being annular and being shown partly broken away with the first member removed;

FIG. 3 is an end view showing the operational relationship between the ytwo members shown separately in FIGS. 1 and 2;

FIGS. 4 and 5 are'generally similar to FIGS. l and 2 respectively but illustrate members having more than the minimum number of conducting paths;

FIG. 6 illustrates -a semi-annular form for the second member of the device; and

FIG. 7 is an end view showing the operational relationship of` such semi-annular form of second member with the first member.

Referring now to FIGS. l to 7, a position sensing de'- vice embodying the 'invention may comprise a cylindrical first member 1 V(FIG. 1 or 4) andan annular (FIG. 2 or 5) or arcuate (FIG. 6) second member 2 which in use is positioned to embrace the member 1 with a small clearance between them. This positional relationship is shown in FIG. 3 lforati annular second member and FIG. 7 for an arcuate second member. The first member 1 is prdvided on its cylindrical surface with an electrically conductive helical path 3 (FIG. l) or a number of interlaced electrically conductive helical paths 3a, 3b, 3e (FIG. 4). The second member 2 is provided withr three or more similar paths which in FIGS. 2 and 5 are constituted by interlaced helices 4a, 4b on the inner surface of member 2, and in FIG. 6 include, on the inner surface of the member 2, paths 4a', 4b which are parts of interlaced helices, the electrical connection of parts of individual helices being completed by paths 4a, 4b respectively, extending over the outer cylindrical surface of the member 2. Each of the helical paths 4a, 4b (FIGS. 2 and 5) or the part-helical paths 4a', 4b (FIG. 6) has the same or substantially t-he same mechanical lead and is of the same hand as the helix 3 (FIG. l) or helices 3a, 3b (FIG. 4) on the member 1. The mechanical lead of the helices is considerably 4exaggerated in the drawing.

tive couplings exist between the path 3 or paths 3a, 3b

3 on the member land the paths 411,4b or 4a', 4b on the member 2') and are arranged for relavtive longitudinal movement, that is, lengthwise of the axis of the cylindrical member l. Electrical connections (not shown) are made ito terminal 5 (FIG. l) or terminais '511, 5b (FIG. 4) (which may themselves be connected conveniently but not necessarily to the ends 'of the helices) on member l and also to terminals 6a, 6b (FIGS. 2, 5 and 6) on member 2.

.As `relative longitudinal movement takes place between the members i and 2, a cyclical variation of the capacitances between the path or paths 5 and the Vpaths 4 will be obtained and maybe utilised for the purpose of position sensing as follows.

Application to terminals 6 on element '2 of voltages vvarying in the same way with respect to time, that is, having similar waveforms, but 'in generall having dierent relative magnitudes (including negative values), may be used to set up an electric eld influencing the path or paths on element l. lt will be understood by tho-se versed in the art that by appropriate control of therelative magnitudes of the applied voltages the electric eld may be kept at allV times similar in l"form but moved to lany desired Aaxial position. More explicitly, if there are, tor example, n similar, interlaced helical conductors on member 2 and the axial spacing betweenhelicesr is uniform, then 'simultaneous application to successive helices of 'simi-lar voltages having magnitudes relatively and respectively proportional to sin 0, sin (+21r/1i),'sin V(l-l-ln-M) Vsin {H+(n-l)21r/n`} will 'result in an electric iield which has substantially the same form for any value of '6 but which takes up different axi-al positions along member 2 for different values of '0 within the range Zero vto 211- radians. Furthermore, the axial change of position of the field consequent upon a change of 6 is substantially proportional to the amount of that change: 'a change in 0 of 21|- radians results in a change of axial position of the iield equal to the mechanical lead of the helical system. The smallest number of separate helices 4 necessary to )give the effect `described is .three `(FIG. 2) but it may be preferred to make n greater than three (FIGS. and 6) if 'this is more suitable to Ianavailable voltage source or will 'minimise the amount of apparatus required to generate the required voltages from a given vphysical representation of 0.

The open .circuit voltage at terminal 5 of ymember l (FIG. 1) resulting from a Flixed excitation of member VZ (-i.e. 0 being constant) will vary cyclically withuniform relative longitudinal movement of the elements, chang- `ing from a maximum in one sense to a maximum in the .opposite sense and back to amaximum in the first .sense as relative .movement between the `two members goes through a distance equal the pitch of the'helix on member f1. IThere .are two positions in one cycle of change of output where the output is zero; .either of these may be `sensed by suitable electrical null-detecting apparatus Jand considered to deiine the .relative position indicated 'by any particular value of the input 0: 'it is evident that if .0 be .changed then the-null will be .detected at la correspondingly changed relative position.

While one .helix only on `member 1'u`ril1sutiice for the detection of the relative position corresponding .to an electrical input to member y2, it maybe ,preferred to use more than one'helix onmemberl. 'For example, if 'two "helices-symmetrically interlaced'be used (not illustrated) 4"then the 'relative position sought may be deiinedbvzero difference of .the voltages appearing.on'the'helicesgthe benefit gainedby this arrangement would be that electrical interference picked up by .both helices lfrom a source external to ythe apparatus is eliminated fromfthe -null-output or is at least substantially reduced .in value 'relative to the desired signal. It is not essentialthat the `i1wopickup helices be symmetrically interlaced; .a parf'ticular Vform of member 1 which has been used success- 'flly is'illus-trated `diagrammatically in 'FIG.4, where the dierence voltage is detected between any chosen pair of helices 3 and the third helix is maintained at zero potential: in this form of element 1 it is possible to use a circuit which detects the average voltage of the null-detecting pair of helices with respect to the potential of the third helix and so obtain a measure of the sensitivity of the system (electrical output per unit longitudinal clisplacement from null position), this measure being availabie to control the overall sensitivity of the whole systern if so desired. l

Another way of using'the .device ofthe invention would be to apply sinusoidal voltages of constant magnitude but of different phasing tothe helices of member 2, so generating an electric field which moves continuously in an axial direction and thus gives a sinusoidal output ,from the helix or lhelices on member 1 which vis constant in magnitude but which varies substantially uniformly in phase with uniform relative longitudinal .movement of members l and `2; .the `phase ofthe output relative 'to the fixed phases of the input maybe used to sense the relative position of the members. The previous discussion regarding the various numbers of helices which may .be .employed :on either member applies .also .to the system .employing phase-sensing.

Where practicable, it Ais preferred to use an annular form of member 2 such as that shown in FIG. 2 or FIG 5, since .then the full peripheral length of the turns Vin the path or paths 3 on the member 1 is used-in providing coupling between these paths and the paths 4 `on the member 2. With the memberZ completely surrounding the member l, however, the latter could be supported permanently .only adjacent its ends. vShouldit be .desired to avoid this limitation, there may be used instead, -for themember 2, an arcuate, preferably semi-annular, form such as .that of FIG. 6.

In Vthe embodiments Aillustrated the rst member 1 of the vsensingdevice has been shown as 'being rectilinear with its :peripheral surface cylindrical, so that its conductor paths `12`and13 are helical .and thereby define `a single ror multi-start helix the-axis of which is parallel tto Vthe direction .of relative movement (linear) between the vtwo members; `It is also possible, however, lfor the rst member to have some other cross-section such for instance as oval or even rectangular. Moreover it is contemplated that a device Vin accordance with the present invention Vcould be employed for sensing angular displacement, rather `than linear displacement if the rst member l1,

:instead of being rectilinear were made curved or in the instancein controlling small rectilinearmovements from relatively ,large annular movements, or in applying por -rections or deliberate 4distortionsto thelaw'by whichithe input quantity (0) is related withthe linear displacement, such corrections or ldistortions `being 'applied by introducing a predetermined extent ofrotary motion-derived from lthe linear motion asby cam control or other means (not shown). With the members thus arranged lgforrelative rotation about the cylinder -axis,-which isalso the'axis o'f thehelical paths `onthe 4first member, one complete revoluti'on'will lcorrespond to `a Vchange in relative'linear movemenlf equal to the lead of the helix or helices onmemberfl. Whereas 'a device conforming to the invention-has lbeen villustratedas having-'its rstmember 1 o'f cylindricalform with its conductive paths on itsfoutside surface, it istobe understood that the first member'of the device could have ahollow form with its conductive path or paths provided on an inside peripheral surface. The second member of the device would then be accommodated within the first member and be provided with its conductive paths on an outwardly facing surface. Further, whereas member 2 has been illustrated as relativelyr short compared with member 1, this relationship of lengths may be reversed.

In carrying out the invention the parts of the two members of the position sensing device which support the conducting paths may be made substantially completely of insulating material, or may consist of insulating material which provides the surfaces carrying the conducting paths and is combined with or bonded to metallic members for the purpose of providing mechanical strength, support, and/or electrical screening. The conductive paths may be provided yen the members in any convenient way. For instance they may be printed on to the members or may take the form of conductive strips suitably wrapped round them. Again, the paths may be machined from a conductive coating which initially completely covers the insulating support, or the surfaces ymay be grooved in accordance with the desired form of the paths and Aconductors laid or formed therein, in which case, the conductors may, if desired, be embedded in a material filling up the grooves thereby to provide a smooth surface on the member concerned.

In employing a device in accordance with the invention chine tool parts without losing the capacitive coupling ben tween the members of the device, would be attached to one of the parts with its axis parallel to the direction of movement, while the shorter member would be yattached or mechanically coupled to the other part in such positiony as to ensure the required capacitive coupling between the conductive paths of the members throughout the relative movement of the machine tool parts.

Since the capacitance would go through several cycles of variation in a total travel of the machine tool parts, some form of coarse indication or control may have to be provided to determine when the relative positions of the parts lie within a particular range, the device of the invention then determining more precisely the relative positions of the parts within that range. It will be appreciated, however, from analogy with conventional synchro systems, that where the indication ork control is continuous no such coarse indication or control may be necessary.

What I claim is:

1. A position sensing device comprising a first mem# ber having a circumferential surface provided with at least one continuous, electrically conductive path passing round and progressing along the member in a'manner analogous to that of a helixkround a cylinder, together with a second member arranged for relative longitudinal movement with respect to the first member and dening a surface facing and substantially conforming to said surface of the first member over at least a portion of the circumference thereof, which second member is provided with at least three separate continuousr conductive paths at least portions of each of which, located on said surface of the second member and substantially conforming in shape and spacing to corresponding portions from adjacent turns in said path on the first member, are disposed so as to have sensible electrical capacitance between them and the path on the first memf ber, the arrangement being such that relative movement 3. A position sensing device as claimed in claim 1 wherein said circumferential surface of the first member is cylindrical and its conductive path or paths helical.

4. A position sensing device as claimed in claim l, wherein said surface on the second member conforms to that on the first member round the entire circumference of the latter and the conductive paths on the second member extend round the surface in amutually interlaced manner analogous to that of a multiple-start helix round a cylinder.

.5. A position sensing device as claimed in claim' l wehrein the surface of the second member conforms to the circumferential surface of the first member over only part of the circumference of the latter, and each conductive path on the second member comprises serially connected portions substantially conforming in shape and `spacing to those portions of the path or paths on the first member which extend over said part of its circumference.

6. A position `sensing device as claimed in claim 5 f wherein said portions of each path are connected in series by connections taken over the rear of the second member between one end of each such portion and the opposite end of the next portion belonging to the same conductive path.

7. A position sensing device as claimed in claim l wherein said second member has n conductive paths (n being not less than three) to which are applied respective voltages having similar waveforms and magnitudes respectively proportional to sin 0, sin (0-i-21r/n), sin (0+(n-l)21r/n), where 0 is a variable angle equal changes of which correspond to kequal changes of position to be sensed.

` `8. A position sensing device as claimed in claim 1 wherein the conductive paths of the second member have applied Ato them respective sinusoidal votlages of constant magnitude but of different phasing such as to generate an electric eld moving continuously in the longitudinal direction of the member.

9. -A position sensing device as claimed in claim 7 wherein the first member has three mutually interlaced conductive paths one of which is maintained at zero potential.

10. A position sensing device as claimed in claim 8 wherein the first member has three mutually interlaced conductive paths one of which is maintained at zero potential.

References Cited in the file of this patent UNITED STATES PATENTS 1,610,122 Edenburg Dec. 7, 1926 2,147,728 lWintringham Feb. 21, 1939 2,611,964 Buisson Sept. 30, 1952 2,638,578 Piety May 12, 1953 

